Piezoelectric crystal



Oct. 16, 1945. E, SWlCKARD 2,386,916

PIEZOELECTRIC CRYSTAL Filed April 9, 1943 INVENTOR. ,4. E. 5 W/CKABD Patented Oct. 16, 1945 PIEZOELECTRIC CRYSTAL Andrew E. Swickard, Chicago, IlL, assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application April 9, 1943, Serial No. 482,361

7 Claims.

' invention, a piezoelectric crystal of substantially parallelopiped shape may be provided having lead wires attached to either side at a point approximately in the middle of the crystal to create an artificial nodal point and to cause the crystal to be resonant at a low frequency.

Other objects and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the following drawing, wherein:

Fig. 1 shows a perspective view of a piezoelectric crystal mounted in accordance with the present invention, and

Fig. 2 shows diagrammatically the low frequency mode of vibration of a piezoelectric crystal supported in accordance with the present invention.

When a low frequency piezoelectric crystal is required, as in certain types of carrier circuits in telephone apparatus, it has sometimes been the practice to employ a quartz crystal of rather large dimensions and having a flexure mode of vibration. The frequency for a given size of flexurev vibration type crystal, when vibrating in free fiexure, is indicated by the following formula:

rag Yr 21412 L= P wherein 7 equals the frequency in cycles per 'flexure vibration, the natural nodal points occur at the points 4 in the crystal. However, when it is attempted to mount the crystal at these points, it will be found that the crystal has a very low activity when driven, at a low frequency or it may have no activity at all. By mounting such a crystal at a point between these natural nodal- .points, it appears that a new, or what may be called an artificial nodal point is created, and when the crystal is then driven at a low ire quency, it will be found to have a high response or high activity.

In accordance with the presentinvention, a

pair of lead wires 5 may be attached to the center of a CT cut crystal 6, that is, a crystal having one edge parallel to its electrical axis and the other 38' from the optical axis, of substantially square or parallelopiped shape. The other ends of the lead wires are soldered to supporting springs l positioned on either side of the crystal 5 and fixed to oppositely disposed terminals 8, mounted in a supporting block 9 and extending therethrough. The following formula gives the approximate frequency for this type of crystal when mounted in accordance with the present invention and driven at its lowest frequency:

f: K214 P wherein K equals approximately one-half /2). It appears that this low frequency mode of vibration is of the flexure type and that the 11!}- usually low frequency of such crystals is due in part at least to the single point of suspension of kc. per second is obtained. The frequency of this vibration can be varied by changing the plate thickness and width as shown in the second formula above.

A crystal of this type, mounted as described herein, is substantially more active and, consequently, more efficient than the standard low frequency X cut crystal of the same frequency, that is, a crystal having one edge perpendicular to the electrical axis and the other parallel to the optical axis. In addition, it may be, as indicated in the formula given above, approxi mately one-half .4;) the sixe of an X out crystal of the same frequency. This reduction in size is a considerable advantage where one or more such crystals are to be used since it reduces space required in the apparatus for housing Because of this reduction in size, the cost of 5 crystals is substantia reduced as compared w other crystals such he X cut of .ailar f- 51116316? range.

quency vibration at such area reiative to its thickness as to have free vibration nodal points spaced along one dimension, and a support for said crystal substantially midway between said nodal points whereby an artificial nodal point is formed causing the crystal to vibrate at a low frequency.

2. A CT cut piezoelectric crystal of such area relative to its thickness as to have a low frequency mode of vibration supported at a single point on either side and vibrating in fiexure.

3. A piezoelectric crystal having a single sup= port fixed to either side thereof, and having a frequency of vibration indicated by the formula W Yo a Ian p I 4. A CT cut piezoelectric crystal supported at an artificial nodal point and having such area relative to its thickness that when driven at a ageeaeie relative to its thickness that when driven at a frequency on the order of 21 kc. it vibrates in a flexure mode of vibration.

6. A CT cut piezoelectric crystal of such area relative to its thickness as to have a low frequency mode of vibration supported at a single point on either side and vibrating in fiexure.

' 7. A CT cut piezoelectric crystalhaving a single support fixed to either side thereof and having a frequency of vibration indicated by the formula ANDREW E. SWICKARD. 

